It turns out it's not really so mysterious after all. The Danger Room interviewed General Montgomery Meigs, who heads up the Pentagon's Joint IED Defeat Organization (JIEDDO). General Meigs admirably--and honestly--refuses credit for the decline:

“I would love to be able to say to the American taxpayer, '40 percent of the reduction in IED incidents and 80 of the reduction in IED effect is due to the things we’ve put in the field.' I can’t get the data that would let me make any kind of an assessment of that.”

So what is the explanation? Meigs reels off the numbers for Noah Shachtman:

What Meigs was able to share, however, were statistics on the number of tips locals gave to coalition forces in Iraq – and number of IED caches found by those troops. As you might expect, there’s a heavy correlation between the two. About 8,500 tips came in September of 2006; by May, the number had peaked at more than 24,000. In August, the figure was approximately 19,200. Similarly, the number caches found – about five per day in September, 2006 – jumped to more than 20 per day in May. After a dip over the early summer, that figure has been steady in recent months, at about 15.

Obviously a major contributing factor to this increase in tips is that the Sunni tribes have turned on al Qaeda--once restive Anbar province hasn't seen an Americans killed by IED since September 10. But there's more to it than that. As Shachtman explains, "A centerpiece of the new American counterinsurgency strategy in Iraq has been to put move combat troops out of local bases – and have ‘em hang out with the locals, instead."

The Petraeus strategy is working.